U.S. patent number 8,742,024 [Application Number 13/128,558] was granted by the patent office on 2014-06-03 for mixture of surface postcrosslinked superabsorbers with different surface postcrosslinking.
This patent grant is currently assigned to BASF SE. The grantee listed for this patent is Stefan Bruhns, Thomas Daniel, Dieter Hermeling, Ulrich Riegel. Invention is credited to Stefan Bruhns, Thomas Daniel, Dieter Hermeling, Ulrich Riegel.
United States Patent |
8,742,024 |
Bruhns , et al. |
June 3, 2014 |
Mixture of surface postcrosslinked superabsorbers with different
surface postcrosslinking
Abstract
A mixture of superabsorbents having differing surface
postcrosslinking, more particularly a mixture of differingly
surface-postcrosslinked sieve cuts of a base polymer, exhibits
improved absorption and retention over a unitarily
surface-postcrosslinked superabsorbent.
Inventors: |
Bruhns; Stefan (Mannheim,
DE), Daniel; Thomas (Waldsee, DE),
Hermeling; Dieter (Bohl-Iggelheim, DE), Riegel;
Ulrich (Landstuhl, DE) |
Applicant: |
Name |
City |
State |
Country |
Type |
Bruhns; Stefan
Daniel; Thomas
Hermeling; Dieter
Riegel; Ulrich |
Mannheim
Waldsee
Bohl-Iggelheim
Landstuhl |
N/A
N/A
N/A
N/A |
DE
DE
DE
DE |
|
|
Assignee: |
BASF SE (Ludwigshafen,
DE)
|
Family
ID: |
41466790 |
Appl.
No.: |
13/128,558 |
Filed: |
November 12, 2009 |
PCT
Filed: |
November 12, 2009 |
PCT No.: |
PCT/EP2009/065036 |
371(c)(1),(2),(4) Date: |
May 10, 2011 |
PCT
Pub. No.: |
WO2010/057823 |
PCT
Pub. Date: |
May 27, 2010 |
Prior Publication Data
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|
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Document
Identifier |
Publication Date |
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US 20110224379 A1 |
Sep 15, 2011 |
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Foreign Application Priority Data
|
|
|
|
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Nov 21, 2008 [EP] |
|
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08169670 |
|
Current U.S.
Class: |
525/327.2;
525/337; 525/526; 525/55; 525/520; 525/327.4; 525/327.3;
525/326.1 |
Current CPC
Class: |
A61L
15/60 (20130101); C08J 3/245 (20130101); C08J
2300/14 (20130101) |
Current International
Class: |
C08F
8/32 (20060101); C08F 8/00 (20060101); C08F
8/42 (20060101); C08G 61/08 (20060101); C09D
175/06 (20060101) |
Field of
Search: |
;524/486
;525/54.2,327.2-327.4,326.1,520,337,526,55 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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102005036992 |
|
Feb 2007 |
|
DE |
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0691133 |
|
Jan 1996 |
|
EP |
|
Other References
Buchholz, Fredric L., et al,. Modern Superabsorbent Polymer
Technology, "Commercial Processes for the Manufacture of
Superabsorbent Polymers," pp. 69-117. New York: John Wiley &
Sons, Inc., 1998. cited by applicant .
International Search Report in PCT Application No.
PCT/EP2009/065036, dated Jan. 21, 2010. cited by applicant.
|
Primary Examiner: Eashoo; Mark
Assistant Examiner: Lacap; Marilou
Attorney, Agent or Firm: Marshall, Gerstein & Borun
LLP
Claims
We claim:
1. A process for producing a mixture of superabsorbents having
differing surface postcrosslinking by mixing differingly
surface-postcrosslinked superabsorbents, wherein different sieve
fractions of a base polymer are separately admixed with a
surface-postcrosslinking agent, subsequently
surface-postcrosslinked by differing heat-treatment duration, and
mixed, wherein the heat treatment utilizes a continuously conveying
and heated dryer and the different base polymer sieve fractions
endowed with surface postcrosslinking agent are fed in at various
points of the dryer.
2. A process for producing a mixture of superabsorbents having
differing surface postcrosslinking by mixing differingly
surface-postcrosslinked superabsorbents, wherein an aqueous
solution of a monomer mixture comprising: a) at least one
ethylenically unsaturated acid-functional monomer which optionally
is at least partly present as a salt, b) at least one crosslinker,
c) at least one initiator, d) optionally one or more ethylenically
unsaturated monomer copolymerizable with the monomer mentioned
under a), e) optionally one or more water-soluble polymer, is
polymerized, the polymer obtained is dried, comminuted, classified
into at least two sieve cuts, wherein the at least two sieve cuts
are differingly surface-postcrosslinked with a
surface-postcrosslinking agent, mixed, and then fed in at various
points of a continuously conveying and heated dryer for a
subsequent heat treatment.
3. A process for producing articles for absorbing fluid, which
comprises incorporating a superabsorbent mixture prepared by the
process of claim 1 in the article.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
This is the U.S. national phase application of International
Application No. PCT/EP2009/065036, filed Nov. 12, 2009, which
claims the benefit of European patent application No. 08169670.0,
filed Nov. 21, 2008.
The present invention relates to a mixture of
surface-postcrosslinked superabsorbents having differing surface
postcrosslinking. More particularly, the present invention relates
to a mixture of superabsorbents of differing average particle size,
which are differingly surface-postcrosslinked. The present
invention further relates to a process for producing such a mixture
and also to its use and to hygiene articles comprising such a
mixture.
Superabsorbents are known. Materials of this type are also commonly
known by designations such as "high-swellability polymer"
"hydrogel" (often even used for the dry form), "hydrogel-forming
polymer", "water-absorbing polymer", "absorbent gel-forming
material", "swellable resin", "water-absorbing resin",
"water-absorbing polymer" or the like. The materials in question
are crosslinked hydrophilic polymers, more particularly polymers
formed from (co)polymerized hydrophilic monomers, graft
(co)polymers of one or more hydrophilic monomers on a suitable
grafting base, crosslinked ethers of cellulose or of starch,
crosslinked carboxymethylcellulose, partially crosslinked
polyalkylene oxide or natural products that are swellable in
aqueous fluids, examples being guar derivatives, of which
superabsorbents based on partially neutralized acrylic acid are
most widely used. The essential properties of superabsorbents are
their ability to absorb and retain amounts of aqueous fluids
equivalent to many times their own weight, even under moderate
pressure. A superabsorbent which is used in the form of a dry
powder transforms into a gel on taking up liquid, specifically into
a hydrogel when as usual taking up water. Their crosslinking
distinguishes synthetic superabsorbents in an essential and
important way from customary merely thickeners, since the
crosslinking renders the polymers insoluble in water. Soluble
substances would have no utility as superabsorbents. By far the
most important field of use for superabsorbents is to absorb bodily
fluids. Superabsorbents are used for example in diapers for
infants, incontinence products for adults or feminine hygiene
products. Examples of other fields of use are as water-retaining
agents in market gardening, as water storage media for protection
against fire, for fluid absorption in food packaging or, very
generally, for absorption of moisture.
Superabsorbents are capable of absorbing and retaining under
pressure a multiple of their own weight of water. In general, such
a superabsorbent will have a Centrifuge Retention Capacity (CRC,
method of measurement given hereinbelow) of at least 5 g/g,
preferably at least 10 gig and more preferably at least 15 g/g. A
superabsorbent can also be a mixture of chemically different
individual superabsorbents or of components which do not have
superabsorbent properties until they cooperate, so it is less its
chemical composition which makes a superabsorbent but the fact that
it has superabsorbent (superabsorbing) properties.
Not just its absorption capacity is important for a superabsorbent,
but also the ability to retain liquid under pressure (retention,
usually expressed as Absorption Under Load (AUL) or Absorption
Against Pressure (AAP)) and also to transport liquid in the swollen
state (usually expressed as Saline Flow Conductivity (SFC)).
Swollen gel can impair or even block (gel blocking) the
transportation of liquid to as yet unswollen superabsorbent. Good
transportation properties for liquids are possessed for example by
hydrogels having high gel strength in the swollen state. Gels
lacking in strength are deformable under an applied pressure, for
example pressure due to body weight, and clog the pores in the
superabsorbent/cellulose fiber absorbent and so prevent continued
absorption of fluid. Enhanced gel strength is generally obtained
through a higher degree of crosslinking, although this reduces the
absorption capacity of the product. An elegant way to enhance gel
strength is to increase the degree of crosslinking at the surface
of the superabsorbent particle compared with the interior of the
particle. Dried superabsorbent particles having an average
crosslink density are subjected to additional crosslinking in a
thin surface layer of their particles, usually in a surface
postcrosslinking step. Surface postcrosslinking increases the
crosslink density in the surface shell of the superabsorbent
particles, raising their absorbency under load to a higher level.
Whereas absorption capacity decreases in the surface layer of the
superabsorbent particles, their core has an improved absorption
capacity (compared to the shell) owing to the presence of mobile
chains of polymer, so that shell construction ensures improved
fluid transmission without occurrence of the gel-blocking effect.
It is likewise known to produce altogether more highly crosslinked
superabsorbents and to subsequently reduce the degree of
crosslinking in the interior of the particles compared with an
outer shell of the particles.
Processes for producing superabsorbents are also known.
Superabsorbents based on acrylic acid, which are the most common
form of superabsorbent on the market, are produced by free-radical
polymerization of acrylic acid in the presence of a crosslinker
(the "internal crosslinker"), with the acrylic acid being partially
neutralized, typically by addition of alkali, usually aqueous
sodium hydroxide solution, before, after or partly before, partly
after the polymerization. The polymer gel thus obtained is
comminuted (which, depending on the polymerization reactor used,
can take place concurrently with the polymerization) and dried. The
dry powder thus obtained (the "base polymer") is typically
postcrosslinked at the surface of the particles by reacting it with
further crosslinkers such as, for example, organic crosslinkers or
multivalent cations, for example aluminum (usually used in the form
of aluminum sulfate), or both, to produce a more highly crosslinked
surface layer compared with the particle interior.
Fredric L. Buchholz and Andrew T. Graham (editors) provide a
comprehensive overview of superabsorbents, their properties and
processes for producing superabsorbents in "Modern Superabsorbent
Polymer Technology", J. Wiley & Sons, New York,
U.S.A./Wiley-VCH, Weinheim, Germany, 1997, ISBN 0-471-19411-5.
EP 691 133 A1 teaches a mixture of superabsorbents having differing
absorption capacity and differing absorption capacity under
pressure. The mixture comprises mixing different
non-surface-postcrosslinked superabsorbents or a
non-surface-postcrosslinked superabsorbent with a
surface-postcrosslinked superabsorbent.
The objective continues to be that of finding new or improved
superabsorbents and processes for producing such superabsorbents.
More particularly, increasing the absorption capacity (CRC) and
also the retention or the absorbency under load (AUL) of the
superabsorbent is a constant objective.
Accordingly, a mixture of superabsorbents having differing surface
postcrosslinking was found. The mixture is notable for higher CRC
and AUL values compared with a unitarily surface-postcrosslinked
superabsorbent. A process for producing such mixtures was also
found, uses of these superabsorbent mixtures and also hygiene
articles comprising these superabsorbent mixtures.
The superabsorbent mixture of the present invention can be produced
by mixing two or more differingly surface-postcrosslinked
superabsorbents using any desired method of mixing. Three, four,
five or any other desired number of differingly
surface-postcrosslinked superabsorbents can also be mixed.
Surface-postcrosslinked superabsorbents per se are known as are
mixing processes.
Differingly surface-postcrosslinked superabsorbents are
superabsorbents which were differingly treated with
surface-postcrosslinking agent in terms of type, amount and/or
aftertreatment and as a result were differingly
surface-postcrosslinked. Nonlimiting examples of differingly
surface-postcrosslinked superabsorbents are, for instance,
superabsorbents endowed with differing amounts of
surface-postcrosslinking agent (in % by weight of
surface-postcrosslinking agent, based on the particular base
polymer), superabsorbents endowed with differing
surface-postcrosslinking agents, or superabsorbents which,
following application of the surface-postcrosslinking agent, were
aftertreated differingly, more particularly at differing
temperature or for differing duration. Superabsorbents differing
only in one such feature, in two or more or in all can be
mixed.
The differingly surface-postcrosslinked superabsorbents can, but
need not, differ in the degree of surface postcrosslinking. The
degree of surface postcrosslinking can be determined indirectly via
the decrease in the CRC of the superabsorbent, since CRC decreases
with the degree of surface postcrosslinking. The increase in the
SFC can also be used as a measure of the degree of surface
postcrosslinking, particularly when further additives influence the
permeability of the swollen superabsorbent.
Mixing can take place after surface postcrosslinking, but also
during surface postcrosslinking. When mixing takes place during
surface postcrosslinking the constituents of the mixture will pass
through part of the surface postcrosslinking conjointly.
Particularly in the case of the commonly used processes for surface
postcrosslinking, which comprise a step for endowing the base
polymer with surface-postcrosslinking agent and a subsequent
heat-treatment step to complete the surface postcrosslinking, the
heat treatment is typically carried out in a heated apparatus which
effects continuous conveyance by commixing. Apparatuses of this
type are frequently used in the chemical industry to dry powders
and they are usually simply referred to as continuous "dryers".
Feeding base polymers separately endowed with
surface-postcrosslinking agent into such a dryer at different
points leads to a mixture of superabsorbents differingly
surface-postcrosslinked as a result of differing heat-treatment
duration at least, and is a particularly convenient method of
producing a mixture which is in accordance with the present
invention. This method of making can additionally utilize different
base polymers, different surface-postcrosslinking agents and/or
different amounts of one or more surface-postcrosslinking agents,
and also dryers having different temperature zones in order that
differing surface postcrosslinking may be created not just through
differing residence time but also through differing
temperature.
In one preferred embodiment of the present invention, the mixture
of superabsorbents having differing surface postcrosslinking
comprises a mixture of differingly surface-postcrosslinked sieve
cuts of a base polymer.
The mixture of the present invention can be essentially a mixture
of differingly surface-postcrosslinked sieve cuts of a base polymer
or else a mixture of differingly surface-postcrosslinked sieve cuts
of a base polymer, i.e., consist of differingly
surface-postcrosslinked sieve cuts of a base polymer.
"Sieve cut" in the context of this invention is to be understood as
meaning a fraction from the entire particle size distribution of a
base polymer. Different sieve cuts differ in average particle size,
which can be determined either by sieve analysis or by optical
methods such as light scattering or laser diffraction. Fractions of
this type are usually recovered by sieving. However, they can also
be obtained by other methods of classification, for instance by
wind sifting including separation in the air stream in cyclones,
although minor secondary effects can arise in such processes, due
to density or particle shape for example, and are routinely taken
into account.
In principle, any desired number of sieve cuts can be present in
the mixture. Preferably, the mixture comprises two, three or four
sieve cuts, more preferably two or three sieve cuts and most
preferably two sieve cuts.
In one preferred embodiment of the present invention, the mixture
comprises at least two different sieve cuts of a base polymer which
were separately endowed with surface-postcrosslinking agent and
then heat-treated for different lengths of time. A particularly
simple way of obtaining such a mixture is to feed the
surface-postcrosslinking agent endowed sieve cuts at various points
of a heated apparatus which effects continuous conveyance by
commixing (a continuous dryer), so that the individual sieve cuts
are heat-treated for differing duration. More preferably, the
mixture of the present invention comprises sieve cuts heat-treated
the longer the smaller their average particle size diameter is.
The superabsorbents present in the mixture of the present invention
are obtainable in different ways, for example by solution
polymerization, suspension polymerization, dropletization or spray
polymerization. Processes of this type are known.
A preferred present-invention polymerization process for producing
acrylate superabsorbents is the aqueous solution polymerization of
a monomer mixture comprising a) at least one ethylenically
unsaturated acid-functional monomer which optionally is at least
partly present as a salt, b) at least one crosslinker, c) at least
one initiator, d) optionally one or more ethylenically unsaturated
monomers copolymerizable with the monomers mentioned under a), and
e) optionally one or more water-soluble polymers.
The monomers a) are preferably water-soluble, i.e., their
solubility in water at 23.degree. C. is typically at least 1 g/100
g of water, preferably at least 5 g/100 g of water, more preferably
at least 25 g/100 g of water and most preferably at least 35 g/100
g of water.
Suitable monomers a) are for example ethylenically unsaturated
carboxylic acids or their salts, such as acrylic acid, methacrylic
acid, maleic acid, maleic anhydride, and itaconic acid or its
salts. Particularly preferred monomers are acrylic acid and
methacrylic acid. Acrylic acid is very particularly preferred.
Further suitable monomers a) are for example ethylenically
unsaturated sulfonic acids, such as styrenesulfonic acid and
2-acrylamido-2-methylpropanesulfonic acid (AMPS).
Impurities can have an appreciable influence on the polymerization.
Therefore, the raw materials used should be very pure. It is
accordingly often advantageous to specially purify the monomers a).
Suitable methods of purification are described for example in WO
2002/055469 A1, WO 2003/078378 A1 and WO 2004/035514 A1. A suitable
monomer a) is for example an acrylic acid purified as described in
WO 2004/035514 A1 to comprise 99.8460% by weiaht of acrylic acid,
0.0950% by weight of acetic acid, 0.0332% by weight of water,
0.0203% by weight of propionic acid, 0.0001% by weight of
furfurals, 0.0001% by weight of maleic anhydride, 0.0003% by weight
of diacrylic acid and 0.0050% by weight of hydroquinone monomethyl
ether.
The proportion of the total amount of monomers a) which is
attributable to acrylic acid and/or salts thereof is preferably at
least 50 mol %, more preferably at least 90 mol % and most
preferably at least 95 mol %.
The monomer solution comprises preferably at most 250 weiaht ppm,
more preferably at most 130 weight ppm and even more preferably 70
weight ppm and also preferably at least 10 weight ppm, more
preferably at least 30 weight ppm and especially around 50 weight
ppm of hydroquinone monoether, all based on the nonneutralized
monomer a), with neutralized monomer a), i.e., a salt of monomer
a), being arithmetically counted as nonneutralized monomer. For
example, the monomer solution is obtainable using an ethylenically
unsaturated acid-functional monomer having an appropriate
hydroquinone monoether content.
Preferred hydroquinone monoethers are hydroquinone monomethyl ether
(MEHQ) and/or alpha-tocopherol (vitamin E).
Suitable crosslinkers b) ("internal crosslinkers") are compounds
having at least two groups suitable for crosslinking. Groups of
this type are for example ethylenically unsaturated groups which
can be free-radically interpolymerized into the polymer chain; and
functional groups capable of forming covalent bonds with the acid
groups of monomer a). Suitable crosslinkers b) further include
polyvalent metal salts capable of forming coordinative bonds with
at least two acid groups of monomer a).
Crosslinkers b) are preferably compounds having at least two
polymerizable groups which can be free-radically interpolymerized
into the polymer network. Suitable crosslinkers b) are for example
ethylene glycol dimethacrylate, diethylene glycol diacrylate,
polyethylene glycol diacrylate, allyl methacrylate,
trimethylolpropane triacrylate, triallylamine, tetraallylammonium
chloride, tetraallyloxyethane as described in EP 530 438 A1, di-
and triacrylates as described in EP 547 847 A1, EP 559 476 A1, EP
632 068 A1, WO 93/21237 A1. WO 2003/104299 A1, WO 2003/104300 A1.
WO 2003/104301 A1 and DE 103 31 450 A1, mixed acrylates which, as
well as acrylate groups, comprise further ethylenically unsaturated
groups, as described in DE 103 31 456 A1 and DE 103 55 401 A1, or
crosslinker mixtures as described for example in DE 195 43 368 A1,
DE 196 46 484 A1, WO 90/15830 A1 and WO 2002/32962 A2.
Preferred crosslinkers b) are pentaerythritol triallyl ether,
tetraallyloxyethane, methylenebismethacrylamide, 10- to 20-tuply
ethoxylated trimethylolpropane triacrylate, 10- to 20-tuply
ethoxylated trimethylolethane triacrylate, more preferably 15-tuply
ethoxylated trimethylolpropane triacrylate, polyethylene glycol
diacrylates having 4 to 30 ethylene oxide units in the polyethylene
glycol chain, trimethylolpropane triacrylate, di- and triacrylates
of 3- to 30-tuply ethoxylated glycerol, more preferably di- and
triacrylates of 10- to 20-tuply ethoxylated glycerol, and
triallylamine. Polyols not fully esterified with acrylic acid can
also be present here as Michael adducts with themselves, in which
case tetra-, penta- or even higher acrylates can also be
present.
Very particularly preferred crosslinkers b) are the diacrylated,
dimethacrylated, triacrylated or trimethacrylated multiply
ethoxylated and/or propoxylated glycerols as described in WO
2003/104301 A1 for example. Di- and/or triacrylates of 3- to
10-tuply ethoxylated glycerol are particularly advantageous. Very
particular preference is given to di- or triacrylates of 1- to
5-tuply ethoxylated and/or propoxylated glycerol. The triacrylates
of 3- to 5-tuply ethoxylated and/or propoxylated glycerol are most
preferable, especially the triacrylate of 3-tuply ethoxylated
glycerol.
The amount of crosslinker b) is preferably in the range from 0.05%
to 1.5% by weight, more preferably in the range from 0.1% to 1% by
weight and most preferably in the range from 0.3% to 0.6% by
weight, all based on monomer a). As crosslinker content increases,
Centrifuge Retention Capacity (CRC) decreases and absorbency under
a pressure of 0.3 psi (AUL 0.3 psi) increases.
Useful initiators c) include any compounds that produce free
radicals under the polymerization conditions, examples being
thermal initiators, redox initiators, photoinitiators. Suitable
redox initiators are sodium peroxodisulfate/ascorbic acid, hydrogen
peroxide/ascorbic acid, sodium peroxodisulfate/sodium bisulfite and
hydrogen peroxide/sodium bisulfite. Preference is given to using
mixtures of thermal initiators and redox initiators, such as sodium
peroxodisulfate/hydrogen peroxide/ascorbic acid. But the reducing
component is preferably a mixture of the sodium salt of
2-hydroxy-2-sulfinatoacetic acid, the disodium salt of
2-hydroxy-2-sulfonatoacetic acid and sodium bisulfite (available as
Bruggolit.RTM. FF6M or Bruggolit.RTM. FF7, alternatively
BRUGGOLITE.RTM. FF6M or BRUGGOLITE.RTM. FF7 from L. Bruggemann K G,
Salzstra.beta.e 131, 74076 Heilbronn, Germany,
www.brueggemann.com).
Ethylenically unsaturated monomers d) copolymerizable with the
ethylenically unsaturated acid-functional monomers a) are for
example acrylamide, methacrylamide, hydroxyethyl acrylate,
hydroxyethyl methacrylate, dimethylaminoethyl methacrylate,
dimethylaminoethyl acrylate, dimethylaminopropyl acrylate,
diethylaminopropyl acrylate, dimethylaminoethyl methacrylate,
diethylaminoethyl methacrylate, maleic acid and maleic
anhydride.
Useful water-soluble polymers e) include polyvinyl alcohol,
polyvinylpyrrolidone, starch, starch derivatives, modified
cellulose, such as methylcellulose or hydroxyethylcellulose,
gelatin, polyglycols or polyacrylic acids, preferably starch,
starch derivatives and modified cellulose.
An aqueous monomer solution is typically used. The water content of
the monomer solution is preferably in the range from 40% to 75% by
weight, more preferably in the range from 45% to 70% by weight and
most preferably in the range from 50% to 65% by weight. It is also
possible to use monomer suspensions, i.e., supersaturated solutions
of monomer. Increasing water content means increasing energy
requirements at the subsequent drying and a decreasing water
content may mean inadequate removal of the heat of
polymerization.
The preferred polymerization inhibitors require dissolved oxygen
for optimum effect. Therefore, the monomer solution can be freed of
dissolved oxygen prior to the polymerization, by inertizing the
monomer solution, i.e., by flowing an inert gas, preferably
nitrogen or carbon dioxide, through it. The oxygen content of the
monomer solution is preferably lowered to less than 1 weight ppm,
more preferably to less than 0.5 weight ppm and most preferably to
less than 0.1 weight ppm prior to the polymerization.
The monomer mixture may comprise further components. Examples of
further components used in monomer mixtures of this type are, for
instance, chelating agents in order that metal ions may be kept in
solution.
Suitable polymerization reactors are for example kneading reactors
or belt reactors. A kneader utilizes for example contrarotatory
stirring shafts to continuously comminute the polymer gel formed in
the polymerization of an aqueous monomer solution or suspension, as
described in WO 2001/38402 A1. Polymerization on a belt is
described in DE 38 25 366 A1 and U.S. Pat. No. 6,241,928 for
example. Polymerization in a belt reactor produces a polymer gel
that has to be comminuted in a further process step, for example in
a mincer, extruder, or kneader. However, it is also possible to
produce spherical particles of superabsorbent via suspension,
sprayed or dropletized polymerization processes.
The acid groups of the polymer gels obtained are typically in a
partly neutralized state. Neutralization is preferably performed at
the monomer stage; that is, salts of the acid-functional monomers
or, to be precise, a mixture of acid-functional monomers and salts
of acid-functional monomers ("partially neutralized acid") are used
in the polymerization as component a). This is typically
accomplished by incorporating the neutralizing agent as an aqueous
solution, or else preferably as a solid, into the monomer mixture
intended for the polymerization, or preferably into the
acid-functional monomer or a solution thereof. The degree of
neutralization is preferably in the range from 25 to 95 mol %, more
preferably in the range from 50 to 80 mol % and most preferably in
the range from 65 to 72 mol %, and the customary neutralizing
agents can be used, preferably alkali metal hydroxides, alkali
metal oxides, alkali metal carbonates or alkali metal bicarbonates
and also mixtures thereof. Instead of alkali metal salts, ammonium
salts can also be used. Sodium and potassium are particularly
preferred for use as alkali metal cations, but sodium hydroxide,
sodium carbonate or sodium bicarbonate and also mixtures thereof
are very particularly preferred.
Neutralization can also be carried out after polymerization, at the
stage of the polymer gel formed in the polymerization. It is
further possible to neutralize up to 40 mol %, preferably from 10
to 30 mol % and more preferably from 15 to 25 mol % of the acid
groups prior to polymerization by adding some of the neutralizing
agent to the monomer solution and setting the desired final degree
of neutralization only after the polymerization, at the polymer gel
stage. When the polymer gel is at least partially neutralized after
polymerization, the polymer gel is preferably subjected to
mechanical comminution, using an extruder for example, in which
case the neutralizing agent can be sprayed, sprinkled or poured on
and then carefully mixed in. To this end, the gel mass obtained can
further be repeatedly extruded for homogenization.
It is preferable, however, to neutralize at the monomer stage. In
other words it is a very particularly preferred embodiment to use
as monomer a) a mixture of 25 to 95 mol %, more preferably from 50
to 80 mol % and most preferably from 65 to 72 mol % of salt of the
acid-functional monomer and a balancing amount (to 100 mol %) of
acid-functional monomer. This mixture is for example a mixture of
sodium acrylate and acrylic acid, or a mixture of potassium
acrylate and acrylic acid.
A preferred embodiment comprises neutralizing using a neutralizing
agent having an iron content of generally below 10 weight ppm,
preferably below 2 weight ppm and more preferably below 1 weight
ppm. It is similarly desirable to have a low level of chloride and
also of anions of oxygen acids of chlorine. A suitable neutralizing
agent is for example the 50% by weight aqueous sodium hydroxide or
potassium hydroxide solution traded as membrane grade, although the
amalgam grade or mercury process grade 50% by weight aqueous sodium
hydroxide or potassium hydroxide solution is purer and preferable,
albeit also costlier.
The polymer gel obtained from the aqueous solution polymerization
with or without subsequent neutralization is then preferably dried
using a belt dryer until the residual moisture content is
preferably in the range from 0.5% to 15% by weight, more preferably
in the range from 1% to 10% by weight and most preferably in the
range from 2% to 8% by weight (see hereinbelow for method of
measuring the residual moisture or water content). When the
residual moisture content is too high, the dried polymer gel will
have an excessively low glass transition temperature Tg and is
difficult to further process. When the residual moisture content is
too low, the dried polym is too brittle and the subsequent
comminuting steps generate undesirably large amounts of polymer
particles of excessively low particle size "fines". The solids
content of the gel before drying is generally in the range from 25%
to 90% by weight, preferably in the range from 30% to 80% by
weight, more preferably in the range from 35% to 70% by weight and
most preferably in the range from 40% to 60% by weight.
Alternatively, a fluidized bed dryer or a heatable mixer having a
mechanical mixing member such as, for example, a paddle dryer or a
similar dryer having differently designed mixing implements can be
used. Optionally, the dryer can be operated under nitrogen or some
other nonoxidizing inert gas or at least under reduced partial
pressure of the oxygen in order that oxidative yellowing processes
may be prevented. Generally, however, sufficient venting and
removal of water vapor also leads to an acceptable product. A very
short drying time is generally advantageous with regard to color
and product quality. To this end, the commonly used belt dryers are
typically operated with the temperature of the drying gas used
being at least 50.degree. C., preferably at least 80.degree. C. and
more preferably at least 100.degree. C. and also generally at most
250.degree. C., preferably at most 200.degree. C. and more
preferably at most 180.degree. C. Commonly used belt dryers often
have a plurality of chambers, and the temperature in these chambers
can differ. For every type of dryer operating conditions must
overall be chosen in a conventional manner such that the desired
outcome is achieved for the drying operation.
Drying also reduces the residual monomer content of the polymer
particles and destroys final traces of the initiator.
The dried polymer gel is subsequently ground and classified, useful
grinding apparatus typically including roll stands having one or
more stages, preferably two or three stages, pin mills, hammer
mills or swing mills. Oversize clumps of gel which are often still
undried in the interior are rubbery, lead to problems at grinding
and preferably are removed before grinding, which can simply be
done by wind sifting or by means of a sieve (acting as a protective
screen for the mill). The mesh size of this protective screen sieve
must be chosen on the basis of the mill used, such that there are
ideally no disruptions due to excessively large, rubbery
particles.
Particles of superabsorbent that are too lame because of
insufficiently fine grinding are noticeable as coarse particles in
hygiene products such as diapers, their predominant use, they also
lower the average rate of swelling of the superabsorbent. Neither
is desirable. It is accordingly advantageous to remove coarsely
granular polymer particles from the product. This is done using
customary methods of classification, for example wind sifting or by
sieving through a sieve having a mesh size of at most 1000 .mu.m,
preferably at most 900 .mu.m, more preferably at most 850 .mu.m and
most preferably at most 800 .mu.m. Sieves of 700 .mu.m, 650 .mu.m
or 600 .mu.m mesh size are used for example. The coarsely granular
polymer particles ("oversize") removed can be recycled to the
grinding and sieving circuit, or further processed separately, for
cost optimization.
Polymer particles that are too small in terms of particle size
decrease permeability (SFC). It is accordingly advantageous to also
remove finely granular polymer particles in the course of this
classification. A convenient way to do this, if sieving is chosen,
is by using a sieve having a mesh size of at most 300 .mu.m,
preferably at most 200 .mu.m, more preferably at most 150 .mu.m and
most preferably at most 100 .mu.m. The removed finely granular
polymer particles ("undersize" or "fines") can be returned to the
monomer stream, the polymerizing gel or the polymerized gel, prior
to drying of the gel, in any desired manner by way of cost
optimization.
The average particle size of the polymer particles removed as a
product fraction is generally at least 200 .mu.m, preferably at
least 250 .mu.m and preferably at least 300 .mu.m and also
generally at most 600 .mu.m and preferably at most 500 .mu.m. The
proportion of particles having a particle size of at least 150
.mu.m is generally at least 90% by weight, preferably at least 95%
by weight and more preferably at least 98% by weight. The
proportion of particles having a particle size of at most 850 .mu.m
is generally at least 90% by weight, preferably at least 95% by
weight and more preferably at least 98% by weight.
The polymer thus obtained has superabsorbent properties and hence
comes within the term "superabsorbent". Its CRC is typically
comparatively high, while its AUL or SFC is comparatively low. Such
a non-surface-postcrosslinked superabsorbent is often referred to
as "foundational polymer" or "base polymer", to distinguish it from
a surface-postcrosslinked superabsorbent produced therefrom.
The superabsorbent particles are surface postcrosslinked to further
improve their properties, more particularly increase their AUL and
SFC values (reducing the CRC value). Mixing at least two
differingly postcrosslinked superabsorbents leads to the
superabsorbent mixture of the present invention. The foundational
polymers used for surface postcrosslinking can be identical or
different.
In a preferred embodiment of the present invention, the product
fraction of the foundational polymer (i.e., the fraction which is
neither undersize nor oversize) is divided into at least two sieve
cuts or recovered in at least two sieve cuts, which are
subsequently surface-postcrosslinked differingly and mixed together
to form the mixture of the present invention. To this end, the
foundational polymer recovered in a first sieving step can be once
more separated in a second step into two or more sieve cuts, or
concurrently with the removal of over- and/or undersize, the
product fraction can be recovered in a plurality of sieve cuts. As
mentioned, classification need not necessarily be by sieving, but
may take the form of any known method of classification. Sieving is
merely the method which is the most convenient in most cases.
One nonlimiting example of a possible separation into sieve cuts
is, for instance, the recovery of a fraction of 100-850 .mu.m
particle size diameter as a product fraction (i.e., particles that
do not pass through a sieve of 850 .mu.m mesh size are separated
off as oversize and particles that are not retained on a sieve of
150 .mu.m mesh size are separated off as undersize) which is
recovered in two sieve fractions of 100-400 and 400-850 .mu.m
particle size diameter through use of an inter-sieve 400 .mu.m in
mesh size. Similarly, other product fractions and other sieve cuts
are recoverable through use of multiple and/or other
inter-sieves.
Suitable postcrosslinkers are compounds comprising groups capable
of forming bonds with at least two functional groups of the
superabsorbent particles. Suitable surface postcrosslinkers in the
case of the market-dominating superabsorbents based on acrylic
acid/sodium acrylate are compounds comprising groups capable of
forming bonds with at least two carboxylate groups. Preferred
postcrosslinkers are amide acetals or carbamates of the general
formula (I)
##STR00001##
where R.sup.1 is C.sub.1-C.sub.12-alkyl,
C.sub.2-C.sub.12-hydroxyalkyl, C.sub.2-C.sub.12-alkenyl or
C.sub.6-C.sub.12-aryl, R.sup.2 is X or OR.sup.6, R.sup.3 is
hydrogen, C.sub.1-C.sub.12-alkyl, C.sub.2-C.sub.12-hydroxyalkyl,
C.sub.2-C.sub.12-alkenyl or C.sub.6-C.sub.12-aryl or X, R.sup.4 is
C.sub.1-C.sub.12-alkyl, C.sub.2-C.sub.12-hydroxyalkyl,
C.sub.2-C.sub.12-alkenyl or C.sub.6-C.sub.12-aryl, R.sup.5 is
hydrogen, C.sub.1-C.sub.12-alkyl, C.sub.2-C.sub.12-hydroxyalkyl,
C.sub.2-C.sub.12-alkenyl, C.sub.1-C.sub.12-acyl or
C.sub.6-C.sub.12-aryl. R.sup.6 is C.sub.1-C.sub.12-alkyl,
C.sub.2-C.sub.12-hydroxyalkyl, C.sub.2-C.sub.12-alkenyl or
C.sub.6-C.sub.12-aryl and X is a carbonyl oxygen common to R.sup.2
and R.sup.3,
wherein R.sup.1 and R.sup.4 and/or R.sup.5 and R.sup.6 may be
bridged C.sub.2-C.sub.6-alkanediyl, and wherein the abovementioned
radicals R.sup.1 to R.sup.6 may additionally have altogether one to
two free valences, and may be joined via these free valences to at
least one suitable foundational structure,
or polyhydric alcohols, in which case the polyhydric alcohol
preferably has a molecular weight of less than 100 g/mol, more
preferably of less than 90 g/mol, even more preferably of less than
80 g/mol and most preferably of less than 70 g/mol, per hydroxyl
group and also non vicinal, geminal, secondary or tertiary hydroxyl
groups, and polyhydric alcohols are either diols of the general
formula (IIa) HO--R.sup.7--OH (IIa) where R.sup.7 is either an
unbranched dialkyl radical of the formula --(CH.sub.2).sub.n--,
where n is an integer from 3 to 20 and preferably from 3 to 12, and
both the hydroxyl groups are terminal, or R.sup.7 is an unbranched,
branched or cyclic dialkyl radical, or polyols of the general
formula (IIb)
##STR00002## where the radicals R.sup.8, R.sup.9, R.sup.10,
R.sup.11 are each independently hydrogen, hydroxyl, hydroxymethyl,
hydroxyethyloxymethyl, 1-hydroxyprop-2-yloxymethyl,
2-hydroxypropyloxymethyl, methyl, ethyl, n-propyl, isopropyl,
n-butyl, n-pentyl, n-hexyl, 1,2-dihydroxyethyl, 2-hydroxyethyl,
3-hydroxypropyl or 4-hydroxybutyl and altogether 2, 3, or 4,
preferably 2 or 3, hydroxyl groups are present, and not more than
one of R.sup.8, R.sup.9, R.sup.10 and R.sup.11 is hydroxyl,
or cyclic carbonates of the general formula (III)
##STR00003##
where R.sup.12, R.sup.13, R.sup.14, R.sup.15, R.sup.16 and R.sup.17
are each independently hydrogen, methyl, ethyl, n-propyl,
isopropyl, n-butyl, sec-butyl or isobutyl, and n is either 0 or
1,
or bisoxazolines of the general formula (IV)
##STR00004##
where R.sup.18, R.sup.19, R.sup.20, R.sup.21, R.sup.22, R.sup.23,
R.sup.24 and R.sup.25 are each independently hydrogen, methyl,
ethyl, n-propyl, isopropyl, n-butyl, sec-butyl or isobutyl and
R.sup.26 is a single bond, a linear, branched or cyclic
C.sub.2-C.sub.12-dialkyl radical, or a polyalkoxydiyl radical
constructed of from one to ten ethylene oxide and/or propylene
oxide units, as possessed by polyglycol dicarboxylic acids for
example.
Preferred postcrosslinkers of the general formula (II) are
2-oxazolidones, such as 2-oxazolidone and
N-(2-hydroxyethyl)-2-oxazolidone, N-methyl-2-oxazolidone,
N-acyl-2-oxazolidones, such as N-acetyl-2-oxazolidone,
2-oxotetrahydro-1,3-oxazine, bicyclic amide acetals, such as
5-methyl-1-aza-4,6-dioxabicyclo[3.3.0]octane,
1-aza-4,6-dioxa-bicyclo[3.3.0]octane and
5-isopropyl-1-aza-4,6-dioxabicyclo[3.3.0]octane, bis-2-oxazolidones
and poly-2-oxazolidones.
Particularly preferred postcrosslinkers of the general formula (I)
are 2-oxazolidone, N-methyl-2-oxazolidone.
N-(2-hydroxyethyl)-2-oxazolidone and
N-hydroxypropyl-2-oxazolidone.
Preferred postcrosslinkers of the general formula (IIa) are
1,3-propanediol, 1,5-pentanediol, 1,6-hexanediol and
1,7-heptanediol. Further examples of postcrosslinkers of formula
(IIa) are 1,3-butanediol, 1,8-octanediol, 1,9-nonanediol and
1,10-decanediol.
The diols are preferably soluble in water in that the diols of the
general formula (IIa) dissolve in water at 23.degree. C. to an
extent of at least 30% by weight, preferably to an extent of at
least 40% by weight, more preferably to an extent of at least 50%
by weight and most preferably to an extent of at least 60% by
weight, examples being 1,3-propanediol and 1,7-heptanediol. Even
more preference is given to such postcrosslinkers which are liquid
at 25.degree. C.
Preferred postcrosslinkers of the general formula (IIb) are
butane-1,2,3-triol, butane-1,2,4-triol, glycerol,
trimethylobropane, trimethylolethane, pentaerythritol, ethoxylated
glycerol, trimethylolethane or trimethylolpropane each having from
1 to 3 ethylene oxide units per molecule and propoxylated glycerol,
trimethylolethane or trimethyloipropane each having from 1 to 3
propylene oxide units per molecule. Preference is further given to
2-tuply ethoxylated or propoxylated neopentylglycol. Particular
preference is given to 2-tuply and 3-tuply ethoxylated glycerol,
neopentylglycol, 2-ethyl-1,3-propanediol and
trimethylolpropane.
Preferred polyhydric alcohols (IIa) and (IIb) have a 23.degree. C.
viscosity of less than 3000 mPas, preferably less than 1500 mPas,
more preferably less than 1000 mPas, even more preferably less than
500 mPas and at most preferably less than 300 mPas.
Particularly preferred postcrosslinkers of the general formula
(III) are ethylene carbonate and propylene carbonate.
A particularly preferred postcrosslinker of the general formula
(IV) is 2,2'-bis(2-oxazoline).
The preferred postcrosslinkers minimize side and descendent
reactions leading to volatile and hence malodorous compounds. The
superabsorbents obtained using the preferred postcrosslinkers are
therefore odor neutral in the moistened state also.
A single postcrosslinker from the above selection can be used or
any desired mixture of various postcrosslinkers.
The postcrosslinker is generally used in an amount of at least
0.001% by weight, preferably at least 0.02% by weight and more
preferably at least 0.05% by weight and also generally at most 2%
by weight, preferably at most 1% by weight and more preferably at
most 0.3% by weight, for example at most 0.15% by weight or at most
0.095% by weight, all based on the mass of the foundational polymer
(the relevant sieve fraction, for example) endowed therewith.
Postcrosslinking is typically carried out by spraying a solution of
the postcrosslinker onto the dried foundational polymer particle.
After spraying, the postcrosslinker-coated polymer particles are
thermally dried, and the postcrosslinking reaction can take place
both before and during drying. When surface postcrosslinkers having
polymerizable groups are used, surface postcrosslinking can also be
effected by free-radically induced polymerization of such groups
via commonly used free-radical formers or else via high-energy
radiation such as UV light for example. This can take place
concurrently with or instead of the use of postcrosslinkers that
form covalent or ionic bonds with functional groups on the surface
of the foundational polymer particles.
Spraying with the postcrosslinker solution is preferably carried
out in mixers having moving mixing implements, such as screw
mixers, disk mixers, paddle mixers or shovel mixers, or mixers
having other mixing implements. Vertical mixers are particularly
preferred, however. But it is also possible for the postcrosslinker
solution to be sprayed in a fluidized bed. Suitable mixers are
available for example as Pflugschar.RTM. plowshare mixers from
Gebr. Lodige Maschinenbau GmbH, Elsener-Stra.beta.e 7-9, 33102
Paderborn, Germany, or as Schugi.RTM. Fiexomix.RTM. mixers,
Vrieco-Nauta.RTM. mixers or Turbulizer.RTM. mixers from Hosokawa
Micron BV, Gildenstraat 26, 7000 AB Doetinchem. Netherlands.
The spray nozzles which can be used are not subject to any
limitation. Suitable nozzles and atomization systems are described,
for example, in the following references: Zerstauben von
Flussigkeiten, Expert-Verlag, Vol. 660, Reihe Kontakt &
Studium, Thomas Richter (2004) and also in Zerstaubungstechnik,
Springer-Verlag, VDI-Reihe, Gunter Wozniak (2002). Mono- and
polydisperse spraying systems can be used. Among the polydisperse
systems, one-material pressurized nozzles (jet- or
lamellae-forming), rotational atomizers, two-material atomizers,
ultrasound atomizers and impingement nozzles are suitable. In the
case of the two-material atomizers, the liquid phase can be mixed
with the gas phase either internally or externally. The spray
profile of the nozzles is uncritical and may assume any desired
form, for example a round jet, flat jet, wide angle round beam or
circular ring spray profile. It is advantageous to use a
nonoxidizing gas when two-material atomizers are used, particular
preference being given to nitrogen, argon or carbon dioxide. The
liquid to be sprayed can be supplied to such nozzles under
pressure. The liquid to be sprayed can be atomized by decompressing
it in the die bore on attainment of a particular minimum velocity.
In addition, it is also possible to use one-material nozzles for
the purposes of the present invention, for example slot dies or
impingement chambers (full-cone nozzles) (for example from
Dusen-Schlick GmbH, Germany, or from Spraying Systems Deutschland
GmbH, Germany). Such nozzles are also described in EP 0 534 228 A1
and EP 1 191 051 A2.
The postcrosslinkers are typically used in the form of an aqueous
solution. When water only is used as the solvent, a surfactant or
deagglomeration assistant is advantageously added to the
postcrosslinker solution or to the foundational polymer itself.
This improves the wetting performance and reduces the tendency to
form clumps.
Any anionic, cationic, nonionic and amphoteric surfactants are
useful as deagglomeration assistants, preference is given to
nonionic and amphoteric surfactants for skin compatibility reasons.
The surfactant may also comprise nitrogen. For example, sorbitan
monoesters, such as sorbitan monococoate and sorbitan monolaurate,
or ethoxylated variants thereof, for example Polysorbat 20.RTM.,
are added. Suitable deagglomeration assistants further include the
ethoxylated and alkoxylated derivatives of 2-propylheptanol which
are sold under the Lutensol XL.RTM. and Lutensol XP.RTM. brands
(BASF SE, Carl-Bosch-Stra.beta.e 38, 67056 Ludwigshafen,
Germany).
The deagalomeration assistant can be metered in separately or added
to the postcrosslinker solution. Preferably, the deagglomeration
assistant is simply added to the postcrosslinker solution.
The amount of the deagglomeration assistant used, based on
foundational polymer, is for example in the range from 0% to 0.1%
by weight, preferably in the range from 0% to 0.01% by weight and
more preferably in the range from 0% to 0.002% by weight. The
deagglomeration assistant is preferably dosed such that the surface
tension of an aqueous extract of the swollen foundational polymer
and/or of the swollen postcrosslinked superabsorbent at 23.degree.
C. is at least 0.060 N/m, preferably at least 0.062 N/m, more
preferably at least 0.065 N/m and advantageously at most 0.072
N/m.
The aqueous postcrosslinker solution, in addition to the at least
one postcrosslinker, may further comprise a cosolvent. The content
of nonaqueous solvent and/or total solvent quantity can be used to
adjust the penetration depth of the postcrosslinker into the
polymer particles. Industrially highly suitable cosolvents are
C1-C6-alcohols, such as methanol, ethanol, n-propanol, isopropanol,
n-butanol, sec-butanol, tert-butanol or 2-methyl-1-propanol,
C2-C5-diols, such as ethylene glycol, 1,2-propylene glycol or
1,4-butanediol, ketones, such as acetone, or carboxylic esters,
such as ethyl acetate. The disadvantage with some of these
cosolvents is that they do have characteristic odors.
The cosolvent itself is ideally not a postcrosslinker under the
reaction conditions. However, in the limit and depending on
residence time and temperature, the cosolvent may end up
contributing to crosslinking to some extent. This will be
particularly the case when the postcrosslinker is relatively inert
and therefore is itself able to form its cosolvent, as with the use
for example of cyclic carbonates of the general formula (IV), diols
of the general formula (IIIa) or polyols of the general formula
(IIIb). Such postcrosslinkers can also be used as a cosolvent when
admixed with more reactive postcrosslinkers, since the actual
postcrosslinking reaction can then be carried out at lower
temperatures and/or shorter residence times than in the absence of
the more reactive crosslinker. Since the cosolvent is used in
relatively large amounts and will also remain in the product to
some extent, it must not be toxic.
The diols of the general formula (IIa), the polyols of the general
formula (IIb), and also the cyclic carbonates of the general
formula (III) are also useful as cosolvents in the process of the
present invention. They perform this function in the presence of a
reactive postcrosslinker of the general formula (I) and/or (IV)
and/or of a di- or triglycidyl compound. However, preferred
cosolvents in the process of the present invention are more
particularly the diols of the general formula (IIa), more
particularly when the hydroxyl groups are sterically hindered by
neighboring groups from participating in a reaction. Such diols are
in principle also useful as postcrosslinkers, but for this require
distinctly higher reaction temperatures or possibly higher use
levels than sterically unhindered diols.
Particularly preferred combinations between a not very reactive
postcrosslinker being used as a cosolvent on the one hand and a
reactive postcrosslinker on the other are combinations of preferred
polyhydric alcohols, diols of the general formula (IIa) and polyols
of the general formula (IIb) with amide acetals or carbamates of
the general formula (I).
Suitable combinations are for example 2-oxazolidone/1,2-propanediol
and N-(2-hydroxyethyl)-2-oxazolidone/1,2-propanediol and also
ethylene glycol diglycidyl ether/1,2-propanediol.
Very particularly preferred combinations are
2-oxazolidone/1,3-propanediol and
N-(2-hydroxyethyl)-2-oxazolidone/1,3-propanediol.
Further preferred combinations are those with ethylene glycol
diglycidyl ether or glycerol diglycidyl or triglycidyl ether with
the following solvents, cosolvents or cocrosslinkers: isopropanol,
1,3-propanediol, 1,2-propylene glycol or mixtures thereof.
Further preferred combinations are those with 2-oxazolidone or
(2-hydroxyethyl)-2-oxazolidone in the following solvents,
cosolvents or cocrosslinkers: isopropanol, 1,3-propanediol,
1,2-propylene glycol, ethylene carbonate, propylene carbonate or
mixtures thereof.
The concentration of cosolvent in the aqueous postcrosslinker
solution is frequently in the range from 15% to 50% by weight,
preferably in the range from 15% to 40% by weight and more
preferably in the range from 20% to 35% by weight, based on the
postcrosslinker solution. In the case of cosolvents having but
limited miscibility with water, it will be advantageous to adjust
the aqueous postcrosslinker solution such that there is only one
phase, if necessary by lowering the concentration of cosolvent.
There is a preferred embodiment where no cosolvent is used. The
postcrosslinker is then only employed as a solution in water with
or without an added deagglomeration assistant.
The concentration of the at least one postcrosslinker in the
aqueous postcrosslinker solution is typically in the range from 1%
to 20% by weight, preferably in the range from 1.5% to 10% by
weight and more preferably in the range from 2% to 5% by weight,
based on the postcrosslinker solution.
The total amount of postcrosslinker solution based on foundational
polymer is typically in the range from 0.3% to 15% by weight and
preferably in the range from 2% to 6% by weight.
The actual surface-postcrosslinking by reaction of the surface
postcrosslinker with functional groups at the surface of the
foundational polymer particles is usually carried out by heating
the foundational polymer wetted with surface postcrosslinker
solution, which is typically referred to as "drying" (but must not
be confused with the above-described drying of the polymer gel from
the polymerization, in which typically very much more liquid has to
be removed). The drying can be effected in the mixer itself, by
heating the jacket, via heat exchange surfaces or by blowing with
warm gases. Simultaneous admixing of the superabsorbent with
surface postcrosslinker and drying can take place in a fluidized
bed dryer for example. But drying is usually carried out in a
downstream dryer, for example a tray dryer, a rotary tube oven, a
paddle or disk dryer or a heatable screw. Suitable dryers are
available for example as Solidair.RTM. or Torusdisc.RTM. dryers
from Bepex International LLC, 333 N.E. Taft Street. Minneapolis,
Minn. 55413, USA, or as paddle or shovel dryers or else as moving
bed dryers from Nara Machinery Co., Ltd. Zweigniederlassung Europa,
Europaallee 46, 50226 Frechen, Germany.
It is possible to heat the polymer particles via contact surfaces
in a downstream dryer, or via a feed of hot warm inert gas, or via
a mixture of one or more inert gases with steam, or only with steam
alone, for drying and surface postcrosslinking. When the heat is
supplied via contact surfaces, it is possible to conduct the
reaction under inert gas at slight or complete underpressure. When
steam is used to heat the polymer particles directly, it is
desirable according to the present invention to operate the dryer
at atmospheric pressure or superatmospheric pressure. It can be
sensible in this case to split the postcrosslinking step into a
heating step with steam and a reaction step under inert gas but
without steam. This can be realized in one or more apparatuses.
According to the present invention, the polymer particles can be
heated up with steam while still in the postcrosslinking mixer. The
foundational polymer used can still have a temperature in the range
from 10 to 120.degree. C. from preceding operations, and the
postcrosslinker solution can have a temperature in the range from 0
to 70.degree. C. More particularly, the postcrosslinker solution
can be heated to reduce the viscosity.
Preferred drying temperatures are in the range from 100 to
250.degree. C., preferably in the range from 120 to 220.degree. C.,
more preferably in the range from 130 to 210.degree. C. and most
preferably in the range from 150 to 200.degree. C. The preferred
residence time at this temperature in the reaction mixer or dryer
is preferably at least 10 minutes, more preferably at least 20
minutes and most preferably at least 30 minutes and typically at
most 60 minutes. Typically, the drying is conducted such that the
residual moisture content of the superabsorbent is generally at
least 0.1% by weight, preferably at least 0.2% by weight and more
preferably at least 0.5% by weight, and also generally at most 15%
by weight, preferably at most 10% by weight and more preferably at
most 8% by weight.
Postcrosslinking can take place under normal atmospheric
conditions. By "normal atmospheric conditions" is meant that no
technical precautions are taken to reduce the partial pressure of
oxidizing gases, such as that of atmospheric oxygen, in the
apparatus in which the postcrosslinking reaction predominantly
takes place (the "postcrosslinking reactor", typically the dryer).
However, it is preferable to conduct the postcrosslinking reaction
under reduced partial pressure of oxidizing gases. Oxidizing gases
are substances which, at 23.degree. C., have a vapor pressure of at
least 1013 mbar and act as oxidizing agents in combustion
processes, examples being oxygen, nitrogen oxide and nitrogen
dioxide, especially oxygen. The partial pressure of oxidizing gases
is preferably less than 140 mbar, more preferably less than 100
mbar, even more preferably less than 50 mbar and most preferably
less than 10 mbar. When thermal postcrosslinking is carried out at
ambient temperature. i.e. at a total pressure of around 1013 mbar,
the total partial pressure of the oxidizing gases is determined via
their volume fraction. The fraction of oxidizing gases is
preferably less than 14% by volume, more preferably less than 10%
by volume, even more preferably less than 5% by volume and most
preferably less than 1% by volume.
Postcrosslinking can be carried out under reduced pressure, i.e.,
at a total pressure of less than 1013 mbar. The total pressure is
typically less than 670 mbar, preferably less than 480 mbar, more
preferably less than 300 mbar and most preferably less than 200
mbar. When drying and postcrosslinking are carried out under air
having an oxygen content of 20.8% by volume, the oxygen partial
pressures corresponding to the abovementioned total pressures are
139 mbar (670 mbar), 100 mbar (480 mbar), 62 mbar (300 mbar) and 42
mbar (200 mbar), wherein the respective total pressures are placed
between the parentheses. Another way of lowering the partial
pressure of oxidizing gases is to introduce nonoxidizing gases,
more particularly inert gases, into the apparatus used for
postcrosslinking. Suitable inert gases are substances which are
present in gaseous form at the postcrosslinking temperature and the
given pressure in the postcrosslinking dryer and which, under these
conditions, do not have an oxidizing effect on the constituents of
the drying polymer particles, examples being nitrogen, carbon
dioxide, argon, water vapor, of which nitrogen is preferred. The
inert gas rate is generally in the range from 0.0001 to 10 m.sup.3,
preferably from 0.001 to 5 m.sup.3, more preferably from 0.005 to 1
m.sup.3 and most preferably from 0.005 to 0.1 m.sup.3, based on 1
kg of superabsorbent.
In the process of the present invention, the inert gas not
comprising water vapor can be nozzled into the postcrosslinking
dryer, but it is particularly preferable to add the inert gas to
the polymer particle stream via nozzles in or shortly upstream of
the mixer in which the superabsorbent is admixed with surface
postcrosslinker.
It will be appreciated that cosolvent vapors removed from the dryer
can be recondensed outside the dryer and optionally recycled.
In one embodiment of the process according to the present
invention, at least two superabsorbents which were
surface-postcrosslinked within the framework of the above
description of typical conditions of surface postcrosslinking, yet
differently than each other, are subsequently mixed. In a further,
preferred embodiment of the process according to the present
invention, two or more different sieve cuts of a foundational
polymer are separately endowed with surface-postcrosslinking agent,
conveniently by spraying in a vertical mixer as described above.
This can take place in two or more (depending on the number of
sieve fractions used) concurrently operated mixers, or in
succession in one mixer, and this naturally requires intermediate
storage of sieve cuts endowed with surface-postcrosslinking agent.
Surface-postcrosslinking agent type and amount can be the same or
different for each sieve fraction.
These sieve cuts can be treated separately from each or one
another, each in their own dryer, to perform the
surface-postcrosslinking reaction, and mixed thereafter. In a
further preferred embodiment, these sieve fractions endowed with
surface-postcrosslinking agent, however, are fed into one
continuous dryer at various points thereof.
Continuously conveying dryers are dryers in which the product
stream to be dried is conveyed continuously from the inlet to the
outlet of the dryer. In the process, the contents of the dryer are
preferably also agitated in order that the entire contents may come
into contact with the heating surfaces. In the process, the dryer
contents undergo a certain degree of perhaps even intensive
commixing, in that there will usually also be a certain amount of
backmixing, but crossmixing dominates by far. In other words, the
residence time distribution of the product in the dryer is closer
to the residence time distribution of a flow tube reactor than to
that of a stirred tank reactor. Typically, the backmixing ratio
(i.e., the maximum residence time deviation of 95% by weight of all
the particles introduced into the dryer at the first product feed
point from the average residence time of all the particles
introduced into the dryer at the first product feed point) is not
more than 50%, preferably not more than 40% and more preferably not
more than 30%. Backmixing ratios of not more than 20% are very
particularly preferred. Methods of measuring the backmixing ratio
are known, usually the appearance of a marker substance is tracked
as a function of time. A customary method of measuring the
backmixing ratio in a continuously conveying kneader, that is
directly applicable to continuously conveying dryers, is described
in WO 2006/034806 A1, for example. A backmixing ratio for product
introduced at further feed points can be measured in a similar
manner. Backmixing ratio is influenced by the design, more
particularly the type and arrangement of the conveying implements,
and the operating parameters of the dryer, more particularly the
fill level, and can be adjusted to the desired value--all that is
known. Dryers suitable for the process of the present invention are
particularly disk or paddle dryers or heated screws, preferably
paddle dryers.
In a preferred convenient process for producing a mixture which is
in accordance with the present invention, already surface
postcrosslinker-endowed foundational polymers (which can be but
need not be different sieve cuts of the same foundational polymer)
are introduced into a continuously conveying dryer at different
points thereof. The different feed points into the dryer are spaced
apart from each or one another such that the desired effect is
achieved. In general, these feed points are at least sufficiently
far apart for the difference in the average residence time of the
product streams fed in at neighboring feed points, expressed as a
percentage, to be greater than the backmixing ratio of the product
streams introduced at the two neighboring feed points. A smaller
separation is usually not sensible, since the backmixing will in
effect not produce any differences in the length of the heat
treatment of the individual products added. Preferably, the feed
points are spaced apart such that the difference in the average
residence time of the product streams added at neighboring feed
points, expressed as a percentage, is at least twice the backmixing
ratio of the product streams added at the neighboring feed points
and more preferably they are spaced apart such that this difference
is at least three times as large.
In one simple embodiment, two sieve cuts of a foundational polymer
are separately endowed with surface-postcrosslinking agent, one of
these sieve cuts is added at the start, i.e., at the first product
inlet, of the dryer and the other sieve cut is added halfway
between the start and the product outlet of the dryer. Provided the
product fill level in the dryer is identical along the length of
the dryer (and this can also be arranged differently via the type
and arrangement of the conveying implements for example) and also
the temperature in the dryer is everywhere the same, this ensures
that the second sieve cut added is heat-treated half as intensively
as the first.
It will be appreciated that herein it would be similarly possible
to classify the foundational polymer after it has been endowed with
surface-postcrosslinking agent. However, in purely practical terms,
the simplest method for this--sieving--is usually difficult with
the typically moist polymer powder following endowment with
surface-postcrosslinking agent.
When different sieve cuts of one foundational polymer are used as
foundational polymers endowed with surface-postcrosslinking agent,
and are endowed in the same way with the same amount of
surface-postcrosslinking agent, it is preferable for finer sieve
cuts, i.e., sieve cuts having a lower average particle size, to be
introduced into the dryer at earlier feed points than coarser sieve
cuts. It is similarly preferable for coarser particles to be
endowed with less surface-postcrosslinking agent, by weight, and/or
with a surface-postcrosslinking agent which, for a given amount,
effectuates a lower degree of surface postcrosslinking. The two
measures--less or less-crosslinking surface-postcrosslinking agent
and less intensive heat treatment--can be used applied individually
or combined.
In principle, however, it is also possible to produce a present
invention mixture of differingly surface-postcrosslinked
superabsorbents by choosing different and hence more particularly
differingly reactive surface-postcrosslinking agents and/or
differing amount thereof and subsequent conjoint or separate but
identical heat treatment, for example conjoint passage through one
dryer.
The simplest embodiment of the present invention process for
producing a mixture of differingly surface-postcrosslinked
superabsorbents is to additionally use an inter-sieve in the
customary sieving off of a foundational polymer, i.e. the removal
of over- and undersize, and so to recover the product in the form
of two sieve cuts, a comparatively fine sieve cut and a
comparatively coarse sieve cut, to endow these two sieve cuts
separately with surface-postcrosslinking agent, for example in a
vertical mixer in each case, and to introduce them into a
continuously conveying dryer at two different points thereof. The
further workup then takes place again conjointly in the same way as
for a unitarily surface-postcrosslinked superabsorbent.
In one preferred embodiment of the present invention, polyvalent
cations are applied to the particle surface before, during or after
postcrosslinking in addition to the postcrosslinkers. This is in
principle a further surface-postcrosslinking via ionic, noncovalent
bonds, but is occasionally also referred to as "complexation" with
the metal ions in question, or simply as "coating" with the
substances in question (the "complexing agent").
Polyvalent cations are applied by spraying with solutions of
divalent or more highly valent cations, usually divalent, trivalent
or tetravalent metal cations, but also polyvalent cations such as
polymers formally constructed wholly or partly of vinylamine
monomers, such as partially or completely hydrolyzed polyvinylamide
(so-called "polyvinylamine"), the amine groups of which are
always--even at very high pH--partly protonated to ammonium groups.
Examples of useful divalent metal cations are in particular the
divalent cations of metals of groups 2 (more particularly Mg, Ca,
Sr, Ba), 7 (more particularly Mn), 8 (more particularly Fe), 9
(more particularly Co), 10 (more particularly Ni), 11 (more
particularly Cu) and 12 (more particularly Zn) of the periodic
table of the elements. Examples of useful trivalent metal cations
are more particularly the trivalent cations of metals of groups 3
including the lanthanides (more particularly Sc, Y, La, Ce), 8
(more particularly Fe), 11 (more particularly Au), 13 (more
particularly Al) and 14 (more particularly Bi) of the periodic
table of the elements. Examples of useful tetravalent cations are
more particularly the tetravalent cations of metals of the
lanthanides (more particularly Ce) and also of group 4 (more
particularly Ti, Zr, Hf) of the periodic table of the elements. The
metal cations can be used not only alone but also mixed with each
or one another. The use of trivalent metal cations is particularly
preferred. The use of aluminum cations is very particularly
preferred.
Of the metal cations mentioned, any metal salt sufficiently soluble
in the solvent to be used is suitable. Metal salts with weakly
complexing anions such as, for example, chloride, nitrate and
sulfate, hydrogensulfate, carbonate, bicarbonate, nitrate,
phosphate, hydrogenphosphate or dihydrogenphosphate are
particularly suitable. Preference is given to salts of mono- and
dicarboxylic acids, hydroxyacids, ketoacids and also amino acids or
basic salts. Examples are, preferably, acetates, propionates,
tartrates, maleates, citrates, lactates, malates, succinates. It is
similarly preferable to use hydroxides. The use of
2-hydroxycarboxylic acid salts such as citrates and lactates is
particularly preferred. Examples of particularly preferred metal
salts are alkali and alkaline earth metal aluminates and hydrates
thereof, such as sodium aluminate and its hydrates, alkali and
alkaline earth metal lactates and citrates and hydrates thereof,
aluminum acetate, aluminum propionate, aluminum citrate and
aluminum lactate.
The cations and salts mentioned can be used in pure form or in the
form of a mixture of various cations or salts. The salts used of
the di- and/or trivalent metal cation may comprise further
secondary constituents such as still nonneutralized carboxylic acid
and/or alkali metal salts of neutralized carboxylic acid. Preferred
alkali metal salts are those of sodium, of potassium and of
ammonium. They are typically used in the form of an aqueous
solution prepared by dissolving the solid salts in water or
preferably produced directly as such, which may save drying and
purifying steps. It can also be advantageous to use the hydrates of
the salts mentioned, because they often are quicker to dissolve in
water than the anhydrous salts.
The amount of metal salt used is generally at least 0.001% by
weight, preferably at least 0.01% by weight and more preferably at
least 0.1% by weight, for example at least 0.4% by weight, and also
generally at most 5% by weight, preferably at most 2.5% by weight
and more preferably at most 1% by weight, for example at most 0.7%
by weight, all based on the mass of the foundational polymer.
The salt of the trivalent metal cation can be used as a solution or
suspension. Useful solvents for the metal salts include water,
alcohols, DMF. DMSO and also mixtures thereof. Particular
preference is given to water and water-alcohol mixtures such as
water-methanol, water-1,2-propanediol and water-1,3-propanediol for
example.
The treatment of the foundational polymer with solution of a di- or
more highly valent cation is carried out in the same way as that
with surface postcrosslinker, including the drying step. The
surface postcrosslinker and the polyvalent cation can be spray
dispensed in a conjoint solution or as separate solutions. The
spraying of the metal salt solution onto the superabsorbent
particles can take place not only before but also after surface
postcrosslinking. In one particularly preferred process, the
spraying with the metal salt solution takes place in the same step
as the spraying with the crosslinker solution, in which case the
two solutions can be spray dispensed separately in succession or
concurrently via two nozzles or the crosslinker and metal salt
solutions can be spray dispensed conjointly via one nozzle.
Particularly when a tri- or more highly valent metal cation such as
aluminum is used for complexation, there is the option of also
adding a basic salt of a divalent metal cation or a mixture of such
salts. Basic salts are salts capable of raising the pH of an acidic
aqueous solution, preferably a 0.1N hydrochloric acid. Basic salts
are typically salts of a strong base with a weak acid.
The divalent metal cation of the optional basic salt is preferably
a metal cation of group 2 of the periodic table of the elements,
more preferably calcium or strontium and most preferably
calcium.
The basic salts of the divalent metal cations are preferably salts
of weak inorganic acids, weak organic acids and/or salts of amino
acids, more preferably hydroxides, bicarbonates, carbonates,
acetates, propionates, citrates, gluconates, lactates, tartrates,
malates, succinates, maleates and/or fumarates and most preferably
hydroxides, bicarbonates, carbonates, propionates and/or lactates.
The basic salt is preferably soluble in water. Water-soluble salts
are salts which at 20.degree. C. have a water solubility of at
least 0.5 g of salt per liter of water, preferably at least 1 g of
salt per l of water, more preferably at least 10 g of salt per l of
water, even more preferably at least 100 g of salt per l of water
and most preferably at least 200 g of salt per l of water.
According to the invention, however, it is also possible to use
salts that have this minimum solubility at the spraying temperature
of the spray solution. It can also be advantageous to use the
hydrates of the salts mentioned, because they often are quicker to
dissolve in water than the anhydrous salts.
Suitable basic salts of divalent metal cations are for example
calcium hydroxide, strontium hydroxide, calcium bicarbonate,
strontium bicarbonate, calcium acetate, strontium acetate, calcium
propionate, calcium lactate, strontium propionate, strontium
lactate, zinc lactate, calcium carbonate and strontium
carbonate.
When the solubility in water is insufficient to prepare a sprayable
solution of the desired concentration, dispersions of the solid
salt in its saturated aqueous solution can also be used. Calcium
carbonate, strontium carbonate, calcium sulfite, strontium sulfite,
calcium phosphate and strontium phosphate can also be used as
aqueous dispersions for example.
The amount of basic salt of the divalent metal cation, based on the
mass of the foundational polymer, is typically in the range from
0.001 to 5% by weight, preferably in the range from 0.01 to 2.5% by
weight, more preferably in the range from 0.1 to 1.5% by weight,
even more preferably in the range from 0.1% to 1% by weight and
most preferably in the range from 0.4% to 0.7% by weight.
The basic salt of the divalent metal cation can be used as a
solution or suspension. Examples thereof are calcium lactate
solutions or calcium hydroxide suspensions. Typically, the salts
are sprayed onto the superabsorbent using a water quantity of not
more than 15% by weight, preferably not more than 8% by weight,
more preferably not more than 5% by weight and most preferably not
more than 2% by weight, based on the superabsorbent.
Preferably, an aqueous solution of the basic salt is sprayed onto
the superabsorbent. This can be done with the superabsorbent
mixture of the present invention but also separately for the
individual superabsorbents of the mixture. Conveniently, the basic
salt is added concurrently with the surface-postcrosslinking agent,
the complexing agent or as a further constituent of the solutions
of these agents. For these basic salts, the addition mixed with the
complexing agent is preferred. When the solution of the basic salt
is not miscible with the solution of the complexing agent without
precipitation, the solutions can be sprayed onto the superabsorbent
separately in succession or simultaneously from two nozzles.
The superabsorbent mixture or the individual superabsorbents has or
have optionally also a reducing compound added to it or them.
Examples of reducing compounds are hypophosphites, sulfinates or
sulfites. Preference is given to the addition of a sulfinic acid
derivative, more particularly a compound of formula (V)
##STR00005##
where M is a hydrogen atom, an ammonium ion, a monovalent metal ion
or one equivalent of a divalent metal ion of groups 1, 2, 8, 9, 10,
12 or 14 of the periodic table of the elements; R.sup.27 is OH or
NR.sup.30R.sup.31, where R.sup.30 and R.sup.31 are each
independently H or C.sub.1-C.sub.6-alkyl; R.sup.28 is H or an
alkyl, alkenyl, cycloalkyl or aryl group which optionally bears 1,
2 or 3 substituents which are independently selected from the group
consisting of C.sub.1-C.sub.6-alkyl, OH, O--C.sub.1-C.sub.6-alkyl,
halogen and CF.sub.3; and R.sup.29 is COOM, SO.sub.3M, COR.sup.30,
CONR.sup.30R.sup.31 or COOR.sup.30, where M, R.sup.30 and R.sup.31
are each as defined above or else, when R.sup.28 is aryl which is
optionally substituted as indicated above, is H,
salts thereof or mixtures of such compounds and/or salts
thereof.
In the above formula, alkyl is straight-chain or branched alkyl of
preferably 1-6 and more particularly 1-4 carbon atoms. Examples of
alkyl are methyl, ethyl, n-propyl, isopropyl, n-butyl, t-butyl,
n-hexyl, etc. The same applies to the alkyl in O-alkyl. Alkenyl is
straight-chain or branched alkenyl preferably of 3-8 carbon atoms
and more particularly 3-6 carbon atoms. Allyl is a preferred
alkenyl. Cycloalkyl is more particularly
C.sub.1-C.sub.6-cycloalkyl, in which case cyclopentyl and
cyclohexyl are preferred. Aryl (including in aralkyl) is preferably
phenyl or naphthyl. When aryl is a phenyl and is substituted, it
preferably has two substituents. These are 2- and/or 4-disposed in
particular.
Halogen is F, Cl, Br or I, preferably Cl or Br.
M is preferably an ammonium ion, an alkali metal ion or one
equivalent of an alkaline earth metal ion or of a zinc ion.
Suitable alkali metal ions are in particular sodium and potassium
ions, and suitable alkaline earth metal ions are in particular
magnesium, strontium and calcium ions.
R.sup.27 is preferably hydroxyl or amino.
R.sup.28 is preferably hydrogen or an alkyl or aryl group which may
be substituted as above.
It preferably bears one or two hydroxyl and/or alkoxy
substituents.
R.sup.29 is preferably either COOM or COOR.sup.30 (M and R.sup.30
are each as defined above) or else, when R.sup.27 is aryl which may
be substituted as indicated above, a hydrogen atom.
In one preferred embodiment, the superabsorbent mixture or the
superabsorbents has or have added to it or them compounds of the
above formula (V) where M is an alkali metal ion or one equivalent
of an alkaline earth metal or zinc ion; R.sup.27 is hydroxyl or
amino; R.sup.28 is H or alkyl; and R.sup.29 is COOM or COOR.sup.30,
where when R.sup.29 is COOM, M in this COOM radical is H, an alkali
metal ion or one equivalent of an alkaline earth metal ion and when
R.sup.29 is COOR.sup.30, R.sup.30 is C.sub.1-C.sub.6-alkyl.
In a further preferred embodiment, the superabsorbent mixture or
the superabsorbents has or have added to it or them compounds of
the above formula (V) where M is an alkali metal ion or one
equivalent of an alkaline earth metal ion or zinc ion; R.sup.27 is
hydroxyl or amino; R.sup.28 is aryl which is optionally substituted
as indicated above, more particularly hydroxyphenyl or
C.sub.1-C.sub.4-alkoxyphenyl; and R.sup.29 is hydrogen.
The groups 1 (H, Li, Na, K, Rb, Cs, Fr), 2 (Be, Mg, Ca, Sr, Ba,
Ra), 8 (Fe, Ru, Os), 9 (Co, Rh, Ir), 10 (Ni, Pd, Pt), 12 (Zn, Cd,
Hg) and 14 (C, Si, Ge, Sn, Pb) of the periodic table of the
elements in the current numbering by IUPAC (International Union of
Pure and Applied Chemistry, 104 T.W. Alexander Drive, Building 19,
Research Triangle Park, N.C. 27709, U.S.A., www.iupac.org), the
international organization responsible for nomenclature in the
field of chemistry, correspond to the groups Ia, IIa, IIb, IVa and
VIIIb in the numbering used by CAS (Chemical Abstracts Service,
2540 Olentangy River Road, Columbus, Ohio 43202, U.S.A.,
www.cas.org).
The sulfinic acid derivatives of the above formula (V) can be added
in pure form, but alternatively also in the mixture with the
sulfite of the corresponding metal ion and of the corresponding
sulfonic acid, which results from the preparation of such compounds
in a conventional manner. The preparation of such sulfinic acid
derivatives of the above formula is known and described in WO 99/18
067 A1 for example. They are also common commercial products and
are available for example in the form of mixtures of the sodium
salt of 2-hydroxy-2-sulfinatoacetic acid, the disodium salt of
2-hydroxy-2-sulfonatoacetic acid and sodium bisulfite from L.
Bruggemann K G (Salzstrarse 131, 74076 Heilbronn, Germany,
www.brueggemann.com) under the designations BRUGGOLIT.RTM. FF6M or
BRUGGOLIT.RTM. FF7. alternatively BRUGGOLITE.RTM. FF6M or
BRUGGOLITE.RTM. FF7.
The addition of one or more reducing compounds to the
superabsorbent mixture or superabsorbents is effected in a
conventional manner by adding the compounds in substance, as a
solution or as a suspension in a solvent or suspension medium
during or after the production of the superabsorbent mixture or
superabsorbents. Typically, a solution or suspension of the
reducing compound in water or an organic solvent is used, for
example in an alcohol or polyol or in mixtures thereof. Examples of
suitable solvents or suspension media are water, isopropanol/water,
1,3-propanediol/water and propylene glycol/water, wherein the
mixing mass ratio is preferably in the range from 20:80 to 40:60.
The solution or suspension may have a surfactant added to it. When
reducing compounds are added, they are generally added in an amount
of at least 0.0001% by weight, preferably at least 0.001% by weight
and more preferably at least 0.025% by weight, for example at least
0.1% by weight or at least 0.3% by weight, and also generally at
most 3% by weight, preferably at most 2.5% by weight and more
preferably at most 1.5% by weight, for example at most 1% by weight
or 0.7% by weight, all based on the total weight of the
superabsorbent.
The reducing compound is generally mixed with the superabsorbent
mixture or superabsorbents in exactly the same way as the surface
postcrosslinker solution or suspension applied to the
superabsorbent for surface postcrosslinking. The reducing compound
can be applied to a foundational polymer as a constituent part of
the surface postcrosslinker solution or of one of its components,
i.e., added to the solution of the surface postcrosslinker or one
of its components. The superabsorbent coated with
surface-postcrosslinking agent and reducing compound then passes
through the further process steps necessary for surface
postcrosslinking, for example a thermally induced reaction of the
surface-postcrosslinking agent with the superabsorbent as per the
process of the present invention. This process is comparatively
simple and economical.
When very high stability to discoloration in prolonged storage is
essential, the reducing compound is applied in a separate process
step, preferably after surface postcrosslinking. When the reducing
compound is applied in the form of a solution or suspension, it is
applied to the already surface-postcrosslinked superabsorbent or
the mixture of the present invention in the same way as the
surface-postcrosslinking agent is applied to the foundational
polymer. Usually, but not necessarily, this is followed--just as in
surface postcrosslinking--by heating to dry the superabsorbent
again. However, the temperature setting for this drying is
generally at most 110.degree. C., preferably at most 100.degree. C.
and more preferably at most 90.degree. C. in order that undesired
reactions of the reducing compound may be avoided. The temperature
setting chosen is such that, in view of the residence time in the
drying assembly, the desired water content is achieved for the
superabsorbent or superabsorbent mixture. It is also perfectly
possible--and convenient--to add the reducing compound individually
or together with other customary auxiliaries, for example
dustproofing agents, anti-caking agents or water to remoisten the
superabsorbent, as described hereinbelow for these auxiliaries, for
example in a cooler disposed downstream of the
surface-postcrosslinking stage. The temperature of the polymer
particles in this case is between 0.degree. C. and 190.degree. C.,
preferably less than 160.degree. C., more preferably less than
130.degree. C. even more preferably less than 100.degree. C. and
most preferably less than 70.degree. C. The polymer particles, if
appropriate after coating, are speedily cooled down to temperatures
below the decomposition temperature of the reducing compound.
When surface postcrosslinking and/or treatment with complexing
agent is followed by a drying step, it is advantageous--but not
absolutely necessary--to cool the product after drying. Cooling can
be done continuously or batchwise, conveniently the product is for
this purpose continuously conveyed into a cooler disposed
downstream of the dryer. Any apparatus known for removing heat from
pulverulent solids can be used for this purpose, more particularly
any apparatus mentioned above as drying apparatus, provided it is
operated not with a heating medium but with a cooling medium such
as cooling water for example, so that the walls and also, depending
on the design, the stirrer implements or other heat-exchange
surfaces do not carry heat into the superabsorbent or
superabsorbent mixture but remove it therefrom. Preference is given
to the use of coolers in which the product is agitated, i.e.,
cooled mixers, for example shovel coolers, disk coolers or paddle
coolers. The superabsorbent can also be cooled in a fluidized layer
by blowing with a cooled gas such as cold air. Cooling conditions
are adjusted so as to obtain a superabsorbent having the
temperature desired for further processing. Typically, the average
residence time in the cooler is generally at least 1 minute,
preferably at least 3 minutes and more preferably at least 5
minutes and also generally at most 6 hours, preferably at most 2
hours and more preferably at most 1 hour, and cooling performance
is such that the product obtained has a temperature of generally at
least 0.degree. C., preferably at least 10.degree. C. and more
preferably at least 20.degree. C. and also generally at most
100.degree. C., preferably at most 80.degree. C. and more
preferably at most 60.degree. C.
The surface-postcrosslinked superabsorbent or the mixture is
optionally ground and/or sieved in a conventional manner. Grinding
is typically not necessary here, but it is usually advisable for
product agglomerates or fines to be sieved off to achieve the
desired particle size distribution for the product. Agglomerates
and fines are either discarded or preferably returned at a suitable
point into the process in a conventional manner, agglomerates after
comminution. The particle sizes desired for surface-postcrosslinked
superabsorbents are the same as in the case of foundational
polymers.
Optionally, the superabsorbent particles may additionally, if
desired, be surface coated at every stage of their manufacturing
process with any known coating, such as film-forming polymers,
thermoplastic polymers, dendrimers, polycationic polymers (such as
polyvinylamine, polyethyleneimine or polyallylamine for example),
water-insoluble polyvalent metal salts, for example magnesium
carbonate, magnesium oxide, magnesium hydroxide, calcium carbonate,
calcium sulfate or calcium phosphate, any water-soluble mono- or
polyvalent metal salt known to a person skilled in the art, for
example aluminum sulfate, salts of sodium, of potassium, of
zirconium or of iron, or hydrophilic inorganic particles, such as
clay minerals, fumed silica, colloidal silica sols such as
Levasil.RTM. for example, titanium dioxide, aluminum oxide and
magnesium oxide. Examples of useful alkali metal salts are sodium
sulfate, potassium sulfate, sodium lactate, potassium lactate,
sodium citrate, potassium citrate, sodium sorbate and potassium
sorbate. Additional benefits can be achieved as a result, examples
being reduced caking tendency of the end or intermediate product at
every stage of the manufacturing process, improved processing
properties or a further enhanced ability to transmit liquid (SFC).
When the additives are used in the form of dispersions and applied
by spraying, they are preferably used as aqueous dispersions and it
is preferable to additionally apply a dedusting agent to fix the
additive on the surface of the superabsorbent. The dedusting agent
is then added either directly to the dispersion of the inorganic
pulverulent additive or else it can be added as a separate solution
before, during or after the application of the inorganic
pulverulent additive, by spraying. The most preferable version is
to simultaneously apply postcrosslinker, deduster and pulverulent
inorganic additive by spraying in the postcrosslinking stage. In a
further preferred version of the process, however, the dedusting
agent is added separately in the cooler, for example by spraying
from above, from below or from the side. Particularly suitable
dedusting agents which can also serve to fix pulverulent inorganic
additives to the surface of the superabsorbent particles are
polyethylene glycols having a molecular weight in the range from
400 to 20 000 g/mol, polyglycerol, 3- to 100-tuply ethoxylated
polyols, such as trimethylolpropane, glycerol, sorbitol and
neopentylglycol. Of particular suitability are 7- to 20-tuply
ethoxylated glycerol or trimethylolpropane, for example Polyol TP
70.RTM. (Perstorp, SE). The latter have the particular advantage
that they lower the surface tension of an aqueous extract of the
superabsorbent particles only insignificantly.
It is similarly possible to adjust the superabsorbents of the
present invention to a desired water content by adding water.
Optionally, the superabsorbents of the present invention are
endowed with further addition agents that stabilize against
discoloration. Examples are more particularly known stabilizers
against discoloration, more particularly reducing substances.
Preference among these is given to solid or dissolved salts of
phosphinic acid (H.sub.3PO.sub.2) as well as phosphonic acid
(H.sub.3PO.sub.2) itself. All phosphinates of alkali metals,
including ammonium, and of alkaline earth metals are suitable for
example. Particular preference is given to aqueous solutions of
phosphinic acid that comprise phosphinate ions and also at least
one cation selected from sodium, potassium, ammonium calcium,
strontium, aluminum, magnesium.
Preference is similarly given to salts of phosphonic acid
(H.sub.3PO.sub.3) as well as phosphonic acid (H.sub.3PO.sub.3)
itself. All primary and secondary phosphonates of alkali metals,
including ammonium, and of alkaline earth metals are suitable for
example. Particular preference is given to aqueous solutions of
phosphonic acid that comprise primary and/or secondary phosphinate
ions and also at least one cation selected from sodium, potassium,
calcium, strontium.
All coatings, solids, addition agents and auxiliary substances can
each be added in separate process steps, but usually the most
convenient method of adding them--if they are not added during the
admixing of the foundational polymer with surface-postcrosslinking
agent--is to add them to the superabsorbent in the cooler, for
example by spraying a solution or adding in finely divided solid or
in liquid form.
The superabsorbent mixture of the present invention generally has a
Centrifuge Retention Capacity (CRC) of at least 5 g/g, preferably
at least 10 g/g and more preferably at least 20 g/g. Further
suitable minimum CRC values are for example 25 g/g, 30 g/g or 35
g/g. CRC is typically not above 40 g/g. Atypical CRC range for
surface-postcrosslinked superabsorbents is from 28 to 33 g/g.
The superabsorbent mixture of the present invention typically has
an Absorbency Under Load (AUL 0.7 psi, method of measurement see
hereinbelow) of at least 18 g/g, preferably at least 20 g/g, more
preferably at least 22 g/g, even more preferably at least 23 g/g
and most preferably at least 24 g/g, and typically not above 30
g/g.
The superabsorbent mixture of the present invention further has a
Saline Flow Conductivity (SFC, method of measurement see
hereinbelow) of at least 10.times.10.sup.-7 cm.sup.3 s/g,
preferably at least 30.times.10.sup.-7 cm.sup.3 s/g, more
preferably at least 50.times.10.sup.-7 cm.sup.3 s/g, even more
preferably at least 80.times.10.sup.-7 cm.sup.3 s/g and most
preferably at least 100.times.10.sup.-7 cm.sup.3 s/g, and usually
not above 1000.times.10.sup.-7 cm.sup.3 s/g.
The present invention further provides hygiene articles comprising
superabsorbent mixtures of the present invention, preferably
ultrathin diapers comprising an absorbent layer consisting of 50%
to 100% by weight, preferably 60% to 100% by weight, preferably 70%
to 100% by weight, more preferably 80% to 100% by weight and most
preferably 90% to 100% by weight of the superabsorbent mixture of
the present invention, not counting the envelope surrounding the
absorbent layer, of course.
The superabsorbent mixtures of the present invention are also very
particularly advantageous in the manufacture of laminates and
composite structures as described in US 2003/0181115 and also US
2004/0019342 for example. In addition to the hot-melt adhesives
described in the two references for producing such novel absorbent
structures and, more particularly, the hot-melt adhesive fibers
described in US 2003/0181115, to which the superabsorbent particles
are attached, the superabsorbent mixtures of the present invention
are also useful in the manufacture of completely analogous
structures using UV-crosslinkable hot-melt adhesives marketed as
AC-Resin.RTM. (BASF SE, Carl-Bosch-Stra.beta.e 38, 67056
Ludwigshafen, Germany) for example. These UV-crosslinkable hot-melt
adhesives have the advantage of being processable at as low a
temperature as 120 to 140.degree. C., and hence they are more
compatible with many thermoplastic substrates. A further
significant advantage is that UV-crosslinkable hot-melt adhesives
are generally recognized as very safe by toxicologists and also do
not give rise to outgassings in the hygiene articles. A very
significant advantage in connection with the superabsorbent
mixtures of the present invention is the property of
UV-crosslinkable hot-melt adhesives of not tending to yellow during
processing and crosslinking. This is advantageous particularly when
ultrathin or partly transparent hygiene articles are to be
produced. The combination of superabsorbent mixtures of the present
invention with UV-crosslinkable hot-melt adhesives is therefore
particularly advantageous. Suitable UV-crosslinkable hot-melt
adhesives are described for example in EP 0 377 199 A2, EP 0 445
641 A1, U.S. Pat. No. 5,026,806, EP 0 655 465 A1 and EP 0 377 191
A2 for example.
The superabsorbent mixture of the present invention can also be
used in other technical fields where liquids, more particularly
water or aqueous solutions are absorbed. These fields are for
example storage, packaging, transportation (as constituents of
packaging material for water- or moisture-sensitive articles, for
example for flower transportation, also as protection against
mechanical impacts); animal hygiene (in cat litter); food packaging
(transportation of fish, fresh meat; absorption of water, blood in
fresh fish or meat packs); medicine (wound plasters,
water-absorbing material for burn dressings or other weeping
wounds), cosmetics (carrier material for pharmachemicals and
medicaments, rheumatic plasters, ultrasonic gel, cool gel, cosmetic
thickeners, sun protection); thickeners for oil-in-water and
water-in-oil emulsions; textiles (moisture regulation in textiles,
shoe inserts, for evaporative cooling, for example in protective
clothing, gloves, headbands); chemical engineering applications (as
a catalyst for organic reactions, to immobilize large functional
molecules such as enzymes, as adhesion agent in relation to
agglomerations, heat storage media, filter aids, hydrophilic
component in polymeric laminates, dispersants, superplasticizers);
as auxiliaries in powder injection molding, in building
construction and engineering (installation, in loam-based renders,
as a vibration-inhibiting medium, auxiliaries in tunnel excavations
in water-rich ground, cable sheathing); water treatment, waste
treatment, water removal (deicing agents, reusable sand bags);
cleaning; agritech (irrigation, retention of melt water and dew
deposits, composting additive, protection of forests against
fungal/insect infestation, delayed release of active components to
plants); for firefighting or for fire protection; coextrusion
agents in thermoplastic polymers (for example to hydrophilicize
multilayered films); production of self-supporting film sheet and
of thermoplastic moldings able to absorb water (e.g., rain and dew
water storage films for agriculture; superabsorbent-containing
films for keeping fruit and vegetables fresh which are packed in
moist films; superabsorbent-polystyrene coextrudates, for example
for food packaging such as meat, fish, poultry, fruit and
vegetables); or as a carrier substance in formulations of active
components (pharma, crop protection).
The present invention articles for absorbing fluid differ from
existing ones in comprising the superabsorbent mixture of the
present invention.
The present invention also provides a process for producing
articles for absorbing fluid, more particularly hygiene articles,
which comprises producing the article in question by utilizing the
superabsorbent mixture of the present invention. In other respects,
processes for producing such articles using superabsorbent are
known.
Test Methods
The superabsorbent is tested using the test methods described
hereinbelow.
The hereinbelow described "WSP" standard test methods are described
in: "Standard Test Methods for the Nonwovens Industry", 2005
edition, jointly issued by the "Worldwide Strategic Partners" EDANA
(European Disposables and Nonwovens Association, Avenue Eugene
Plasky, 157, 1030 Brussels, Belgium, www.edana.org) and INDA
(Association of the Nonwoven Fabrics Industry, 1100 Crescent Green,
Suite 115, Cary, N.C. 27518, U.S.A., www.inda.org). This
publication is available both from EDANA and INDA.
Measurements described hereinbelow should all be carried out,
unless otherwise stated, at an ambient temperature of
23.+-.2.degree. C. and a relative humidity of 50.+-.10%. The
superabsorbent particles are efficiently commixed before
measurement, unless otherwise stated.
Centrifuge Retention Capacity (CRC)
The centrifuge retention capacity of the superabsorbent is
determined as per the standard test method No. WSP 241.5-02
"Centrifuge retention capacity".
Absorbency Under Load of 0.7 psi (AUL0.7 psi)
The absorbency under a pressure of 4826 Pa (0.7 psi) of the
superabsorbent is determined similarly to the standard test method
No. WSP 242.2-05 "Absorption under pressure", except that a weight
of 49 g/cm.sup.2 (leading to a pressure of 0.7 psi) is used instead
of a weight of 21 g/cm.sup.2 (leading to a pressure of 0.3
psi).
Saline Flow Conductivity SFC)
The flow conductivity of a swollen layer of gel formed by the
superabsorbent by absorption of a liquid is determined under a
confining pressure of 0.3 psi (2068 Pa) as described in EP 640 330
A1 as the Gel Layer Permeability (GLP) of a swollen gel layer of
superabsorbent particles (referred to there as "AGM" for "absorbent
gelling material"), although the apparatus described in the
aforementioned patent application at page 19 and FIG. 8 is modified
to the effect that the glass frit 40 is no longer used, the piston
39 is made of the same plastics material as the cylinder 37 and now
comprises 21 equally sized holes uniformly distributed over the
entire contact surface. The procedure and evaluation of the
measurement remains unchanged from EP 640 330 A1. Flow rate is
recorded automatically.
Saline flow conductivity (SFC) is computed as follows: SFC
[cm.sup.3s/g]=(Fg(t=0).times.L0)/(d.times.A.times.WP), where
Fg(t=0) is the flow rate of NaCl solution in g/s obtained from a
linear regression analysis of the Fg(t) data of the flow rate
determinations by extrapolation to t=0; L0 is the thickness of the
gel layer in cm; d is the density of the NaCl solution in
g/cm.sup.3; A is the area of the gel layer in cm.sup.2; and WP is
the hydrostatic pressure on the gel layer in dyn/cm.sup.2.
Moisture content of superabsorbent (residual moisture content,
water content)
The water content of the superabsorbent particles is determined as
per the standard test method No. WSP 230.2-05 "Moisture
content".
Average Particle Size
The average particle size of the product fraction is determined as
per the standard test method No. WSP 220.2-05 "Particle size
distribution".
EXAMPLES
Example 1
Preparing a Foundational Polymer (Comparative)
A Pflugschar.RTM. plowshare mixer of the VT 5R-MK type, having a 5
liter capacity equipped with a heating/cooling jacket
(manufacturer: Gebr. Lodige Maschinenbau GmbH; Elsener-Stra.beta.e
7-9, 33102 Paderborn, Germany) was initially charged with a
reaction mixture formed of 183 g of water, 239 g of acrylic acid
and 2148 g of a 37.3% by weight sodium acrylate solution (100 mol %
neutralized) and also 2.8 g of 3-tuply ethoxylated glycerol
triacrylate and inertized for 20 minutes by bubbling nitrogen
therethrough. In the process, the reaction mixture was temperature
controlled such that the subsequent addition of initiator took
place at about 20.degree. C. Under agitation, 2.39 g of sodium
persulfate (dissolved in 13.53 g of water), 0.05 g of ascorbic acid
(dissolved in 10.18 g of water) and 0.14 g of 30% by weight
hydrogen peroxide (dissolved in 1.28 g of water) were rapidly added
to the mixer as initiators in succession. The reaction ensued
speedily. From attainment of an internal temperature of 30.degree.
C. the jacket of the mixer was heated with hot heat transfer medium
at 80.degree. C. After the maximum temperature was reached, cooling
fluid (-12.degree. C.) was used to cool the resulting gel in the
mixer down to below 50.degree. C. and the gel was then discharged.
The gel was spread onto two wire-bottomed trays and dried at
160.degree. C. in a circulating air drying cabinet for 2 hours. The
dried gel was subsequently comminuted using a laboratory
ultracentrifugal mill (manufacturer: Retsch GmbH; Rheinische
Stra.beta.e 36, 42781 Haan, Germany; Type ZM 200). The product was
sieved to recover four product fractions having particle sizes from
150 to 300 .mu.m, from 300 to 400 .mu.m, from 400 to 500 .mu.m and
from 500 to 710 .mu.m.
The AUL 0.7 psi and CRC values of these sieve cuts of a
foundational polymer were:
TABLE-US-00001 fraction AUL 0.7 psi CRC [.mu.m] [g/g] [g/g] 150-300
7.1 34.1 300-400 7.5 34.7 400-500 7.7 35.5 500-710 7.9 35.0
Example 2
Surface-Postcrosslinking the Model Foundational Polymer
(Comparative)
Equal portions of the four sieve cuts from example 1 were combined
to form a model foundational polymer comprising 25% by weight each
of every sieve cut.
1.2 kg of foundational polymer obtained according to the procedure
of example 1 were sprayed with crosslinker solution in a
Pflugschar.RTM. plowshare mixer of type VT 5R-MK having a 5 liter
capacity and equipped with heating/cooling jacket (manufacturer:
Gebr. Lodige Maschinenbau GmbH; Elsener-Stra.beta.e 7-9, 33102
Paderborn, Germany) at room temperature under intensive commixing.
A customary two-material spray nozzle as also used for laboratory
spray dryers was used (manufacturer: Buchi Labortechnik GmbH, Am
Porscheplatz 5, 45127 Essen, Germany). The composition of the
crosslinker solution, based on the foundational polymer used, was:
0.10% by weiaht of N-(2-hydroxyethyl)oxazolidinone, 1.10% by weiaht
of n-propanol and also 2.80% by weight of water. The moist polymer
was then dried in a second Pflugschar.RTM. plowshare mixer of the
same design at a polymer temperature of 185.degree. C. for 60
minutes with a 5 g polymer sample being taken every 10 minutes.
The time course of the development of AUL 0.7 psi, CRC and SFC
during the heat treatment is shown in the table which follows:
TABLE-US-00002 time AUL 0.7 psi CRC SFC [min] [g/g] [g/g]
[10.sup.-7 cm.sup.3 s/g] 0 -- 34.7 -- 10 -- 36.0 -- 20 -- 35.5 --
30 19.5 31.9 5 40 24.3 29.9 27 50 24.5 27.7 43 60 24.3 26.6 65
Example 3
Comparative
Following conclusion of the experimental series of example 2 (i.e.,
after 60 minutes), the polymer was removed and again separated by
sieving into the individual sieve cuts.
The AUL 0.7 psi, CRC and SFC values of these sieve cuts were:
TABLE-US-00003 fraction AUL 0.7 psi CRC SFC [.mu.m] [g/g] [g/g]
[10.sup.-7 cm.sup.3 s/g] 150-300 23.6 25.0 70 300-400 23.9 26.7 72
400-500 24.6 28.5 82 500-710 24.3 28.3 68
Comparison with the corresponding values of the mixture (last line
of the table of example 2) shows that AUL and CRC of the conjointly
surface-postcrosslinked mixture correspond essentially to the mean
of the corresponding values of the sieve cuts, but that the mixture
SFC is determined by the SFC of the sieve cut having the lowest
SFC.
Example 4
A 150-300 .mu.m sieve cut obtained according to example 1 was
surface postcrosslinked as described in example 2 for the mixture.
The time course of the development of AUL 0.7 psi, CRC and SFC
during the heat treatment is shown in the table which follows:
TABLE-US-00004 time AUL 0.7 psi CRC SFC [min] [g/g] [g/g]
[10.sup.-7 cm.sup.3 s/g] 0 7.1 34.1 -- 10 -- 35.5 -- 20 -- 34.5 --
30 17.7 31.5 0 40 21.7 29.1 8 50 24.3 27.7 20 60 24.8 27.0 34
Example 5
A 300-400 .mu.m sieve cut obtained according to example 1 was
surface postcrosslinked as described in example 2 for the mixture.
The time course of the development of AUL 0.7 psi, CRC and SFC
during the heat treatment is shown in the table which follows:
TABLE-US-00005 time AUL 0.7 psi CRC SFC [min] [g/g] [g/g]
[10.sup.-7 cm.sup.3 s/g] 0 7.5 34.7 -- 10 -- 35.6 -- 20 -- 34.5 --
30 20.7 31.4 3 40 24.8 29.0 18 50 25.0 28.0 36 60 24.5 26.9 52
Example 6
A 400-500 .mu.m sieve cut obtained according to example 1 was
surface postcrosslinked as described in example 2 for the mixture.
The time course of the development of AUL 0.7 psi, CRC and SFC
during the heat treatment is shown in the table which follows:
TABLE-US-00006 time AUL 0.7 psi CRC SFC [min] [g/g] [g/g]
[10.sup.-7 cm.sup.3 s/g] 0 7.7 35.5 -- 10 -- 36.9 -- 20 -- 35.4 --
30 24.4 32.3 29 40 25.6 30.0 95 50 25.7 28.7 122 60 24.9 27.6
137
Example 7
A 500-710 .mu.m sieve cut obtained according to example 1 was
surface postcrosslinked as described in example 2 for the mixture.
The time course of the development of AUL 0.7 psi, CRC and SFC
during the heat treatment is shown in the table which follows:
TABLE-US-00007 time AUL 0.7 psi CRC SFC [min] [g/g] [g/g]
[10.sup.-7 cm.sup.3 s/g] 0 7.9 35.0 -- 10 -- 35.4 -- 20 -- 35.4 --
30 25.1 32.5 36 40 26.2 31.2 90 50 25.8 29.1 136 60 25.2 28.1
169
The comparison of examples 4 to 7 shows that the relatively fine
sieve cuts, when subjected to the identical endowment with
surface-postcrosslinking agent, need an appreciably more intensive
heat treatment than the coarser ones to establish permeability.
Accordingly, appropriately adapted surface postcrosslinking of
comparatively fine particles makes it possible to achieve a higher
permeability for the mixture as a whole.
Example 8
The model foundational polymer obtained according to example 1 was
postcrosslinked as described in example 2 except that the heat
treatment was carried out for 50 minutes. Similarly, a sample of
each sieve cut of the foundational polymer was similarly
surface-postcrosslinked and subjected to the heat treatment for the
duration reported in the table which follows. The four
surface-postcrosslinked sieve cuts were subsequently recombined to
form a mixture. The CRC, AUL 0.7 psi and SFC values achieved are
likewise reported in the table which follows.
TABLE-US-00008 duration of heat sample treatment AUL 0.7 psi CRC
SFC (postcrosslinked) [min] [g/g] [g/g] [10.sup.-7 cm.sup.3 s/g]
model 50 24.6 27.9 48 foundational polymer 150-300 .mu.m 70 24.4
25.6 50 300-400 .mu.m 55 25.6 28.5 56 400-500 .mu.m 35 25.2 31.3 60
500-710 .mu.m 32 26.2 32.1 53 mixture of 48 25.6 29.0 48 sieve cuts
(averaged)
In these tests, surface postcrosslinking was carried out such that
the mixture of separately postcrosslinked sieve cuts is subjected
to virtually the same averaged heat-treatment duration, and the
same permeability is achieved, compared with the conjointly
surface-postcrosslinked mixture. There was no optimization for high
permeability. The procedure corresponds to separate endowment of
four sieve cuts of a foundational polymer with identical amounts of
the same surface-postcrosslinking agent and their introduction into
a continuously conveying dryer at four separate points which
correspond to the particular heat-treatment time reported as an
average residence time for the particular sieve cut in the dryer.
These tests show that the process of the present invention thereby,
for the same permeability, provides a higher absorbency for the
mixture of separately postcrosslinked sieve cuts.
* * * * *
References